SALMONIDS. 



45 



The subject, which I now present, is the head of the Silver Trout 

 of Europe, Salmo Lacustris, a species found in the large lakes of that 

 continent. The figure is copied, by permission, from Professor 

 Agassiz' great work on the " Fresh-water Fishes of Central Europe." 



*/*/" 



The gill-covers of all the fishes of the three first divisions, with 

 which alone we have to do, consist of four principal parts, and their use 

 is to close the aperture behind the gills, which in all these three 

 divisions is so formed, and so freely or loosely suspended, that the 

 water bathes in its passage every part of their surface. 



These parts are, the pre-opercuhtm, or fore-gill-cover, No. 1 ; the 

 operculum, or gill-cover proper, No. 2; the sub-operculum, or under- 

 gill-cover, No. 3 ; and the inter-operculwm, or intermediate gill-cover, 

 No. 4. The b'ranchiostegous rays, as they are termed, are indicated 

 by No. 5 ; and the fixed plates, forming the posterior immovable mar- 

 gin of the gill-covers, by No. 6. N. 7. indicates the pectoral fin. 



How widely these parts differ in form, in different species of the 

 Salmon tribe, will become at once apparent by a comparison between 

 the gill-covers in the figure above, and those of the true Salmon, Sal- 

 mo Salar, and. the Bull-Trout, Salmo Eriox, Nos. 2 and 3, on the 

 following cut, which, with these, presents a view of the interior of the 

 mouth and the dental system of the common Trout, Salmo Fario, of 

 Great Britain. 



